1. Field of the Invention
The present invention relates to a rotational magnetic head apparatus. More specifically, the present invention relates to an improvement in the mounting of a magnetic head an upper or middle drum rotating system.
2. Description of the Prior Art
FIGS. 1 and 2 are views showing one example of a conventional rotational head apparatus. Specifically, FIG. 1 shows a perspective view of a conventional rotational head apparatus and FIG. 2 is an enlarged side view showing the vicinity of a magnetic head in the conventional rotational head apparatus.
The conventional rotational magnetic head apparatus comprises a fixed drum 4, a rotational drum 2 of the same diameter as that of the fixed drum 4 rotatably mounted on the fixed drum with the axes aligned, and a magnetic head provided within an opening formed at the end surface of the rotational drum 2 facing the fixed drum 4. A magnetic tape 3 is adapted to travel, with the same wound partly around the periphery of the rotational drum and the fixed drum 4.
The opening 5 formed at the end surface of the rotational drum 2 facing the fixed drum is defined by the uppermost or bottom surface 6 facing the opposing end surface 7 of the fixed drum 4, and the side walls. A guiding offset portion 8 is formed on the outer periphery of the fixed drum 4. The rotational drum 2 is adapted to be rotated in the direction of arrow 9. Travel of the magnetic tape 3 is shown by a broken line and, in the case of the apparatus shown, the direction x indicates the traveling direction of the magnetic tape 3. The direction y shows a direction perpendicular to the travel direction x of the magnetic tape 3. The height or depth of the opening 5 formed in the rotational drum 2 in the direction y is shown by the reference character H and is hereinafter referred to as the opening height in this specification. The distance between the upper edge of the magnetic head 1 and the uppermost surface 6 of the opening 5 facing the fixed drum 4 is denoted by the reference character l.sub.1. The uppermost surface 6 of the opening 5 is hereinafter referred to as the uppermost end surface of the opening in this specification. The distance between the lower end of the magnetic head 1 and the opposing end surface 7 of the fixed drum 4 is denoted by the reference character l.sub.2. The opposing end surface 7 of the fixed drum 4 is hereinafter referred to as the upper end surface of the fixed drum 4. The distance between the lower end surface of the rotational drum 2 facing the fixed drum 4 and the upper end surface 7 of the fixed drum 4 is denoted by the reference character l.sub.3. The opposing end surface of the rotational drum 2 facing the fixed drum 4 is hereinafter referred to as the lower end surface of the rotational drum 2. In conventional apparatuses, it was a common practice that a consideration is given such that the distances l.sub.2 and l.sub.3 may be substantially the same.
Conventionally, the distance l.sub.2 was selected to be within an upper limit of a certain magnitude for the purpose of stabilization of the travel of a magnetic tape. On the other hand, there was no particular restriction as to the distance l.sub.1 and typically the distance l.sub.1 was selected to be as large as more than two times the distance l.sub.2 in conventional examples.
With a conventional rotational magnetic head apparatus of such structure, when the rotational drum 2 is rotated in the direction of the arrow 9, the magnetic tape 3 is caused to travel, with one side end of the tape in contact with the guiding offset portion 8 and the tape 3 partly wound around the periphery of the fixed drum 4 and the rotational drum 2 for a predetermined angle, while the magnetic head 1 is sliding on the surface of the tape 3. Since, the magnetic head 1 slightly protrudes from the outer peripheral surface of the rotational drum 2, deformation is caused in the magnetic tape 3. Investigation of such deformation of the magnetic tape 3 in the vicinity of the magnetic head 1 indicates that the magnetic tape 3 is deformed as shown in FIGS. 3 to 5. Specifically, FIG. 3 is a view showing three dimensionally the deformation of the magnetic tape in the vicinity of the magnetic head of a conventional rotational magnetic head apparatus, FIG. 4 is a view showing in a contour line manner the deformation of the magnetic tape in the vicinity of the magnetic head of a conventional rotational magnetic head apparatus, wherein with the outer peripheral surface of the rotational drum 2 as a reference, convex deformation from the outer periphery surface is shown in a solid line and a concave deformation inward of the outer peripheral surface is shown by a broken line, and FIG. 5 is a graph showing dislocation of the magnetic tape in the sections along the line A--A and the line B--B in FIG. 4. A deformed convex portion 10 protruding at the center shown in FIG. 3 is caused by influence of the magnetic head 1 and the deformation amount is increased or decreased in accordance with the magnitude of the protruding amount of the magnetic head 1 from the outer peripheral surface of the rotational drum 3 and generally exhibits a locally deformed state in which only a portion very close to the magnetic head 1 is deformed. In such a case, a deformed concave portion 11 is caused conversely at the foot area of the deformed convex portion 10 caused by the magnetic head 1. The state of the deformation is converted to a representation in terms of a contour line and the same is shown in FIG. 4, together with the indication of the positions of the respective corresponding drums. In the figure, the solid line in the center shows a contour line of the deformed convex portion 10 of the magnetic tape 1 and the dotted line shows the deformed concave portion 11 of the magnetic tape 1, and a two dotted line shows a contour lines of the rotation drum 2 and the fixed drum 4 corresponding to FIG. 2. It is seen that in such a case, the central points of these deformations do not appear on one line but, as shown in FIG. 4, two deformed concave portions 11 are formed in the portions close to the upper end of the opening 5 of the rotational drum 2.
As a result, the contour line of the deformed convex portion 10 is distorted at the right upper portion and the left upper portion of the contour line due to an influence by the deformed concave portions 11, with the result that an accurate (an elliptical) arch is not formed. In order to investigate in more detail a mutual positional relation of the deformation of the magnetic tape 3 with respect to the magnetic head 1, the dislocation of the magnetic tape at the positions of the sections shown by one dotted lines A--A and B--B in FIG. 4 was investigated and the result is shown in FIG. 5. The deformation at the section A--A is shown by the curve a and the deformation at the section B--B is shown by the curve b. The measuring point in the abscissa was determined in the direction y with the position of the magnetic head 1 as the origin 5. It would be appreciated from the curve b that the deformed state of the deformed convex portion 10 is asymmetrical as to the rotational drum side (the right side as viewed in FIG. 5) and as to the fixed drum side (the left side as viewed in FIG. 5), with the position of the magnetic head 1 being the most protruding portion as a reference. This shows that a contact state of the magnetic tape 3 with the magnetic head 1 is asymmetrical and, considering the fact that the track position (the gap position) on the magnetic head 1 comes approximately in the center of the magnetic head and the track width is as small as several tens .mu.m, degradation of a recording and reproducing state has been caused due to the so called "unbalanced contact". However, such degradation is not preferred.
FIG. 6 is a diagrammatic view showing a contact state of the magnetic tape with the magnetic head. More specifically, since the distance l.sub.1 as compared with the distance l.sub.2 is too large, deformation of the magnetic tape 3 becomes uneven in the y axis direction (the height direction) with respect to the magnetic head 1 and therefore the spacing between the magnetic tape 3 and the magnetic head 1 becomes large at the position of the head track 12, as shown in FIG. 6, with the result that a recording and reproducing state is degraded. Conventionally, such problems were solved by improving the shape of the end of the magnetic head 1; however, control of the end shape of the magnetic head is extremely difficult and expensive.